This proposal will examine the importance of matrix metalloproteinase-9 (MMP-9) as a regulator of intimal lesion formation. The rationale for this study is that migration of SMC into the new neointima is a critical step in arterial lesion formation. This hypothesis is now supported by our data showing a significant reduction in the size of the neointima in MMP-9 -/- as compared to wild type arteries (FVB background). The experiments of the first aim will validate if this phenotypic change is influenced by the mouse background strain. SMC replication and migration will be analyzed in MMP-9 -/- arteries on a C57BL/6 background. Studies will also determine if direct inhibition of MMP-9 in wild type cells (FVB) will retard migration and replication. Aim 2 will ask if activation of MMP-9 requires plasmin activity. SMC from plaminogen -/- mice will be stimulated to express MMPs and the ratio of inactive and active MMP-9. In a similar manner, the migration of SMC from plaminogen -/- and of wild type arteries will be measured in vitro. A direct role for plasmin activity regulating SMC migration will be tested by transfecting MMP-9 -/- SMC with an adenoviral construct expressing uPA. The next aim will explore how MMP-9 regulates SMC replication. Experiments will determine if MMP-9 regulates the release of arterial growth factors from the extracellular matrix. The activation of ERK and PI3K signal transduction pathways will be measured in injured wild type and MMP-9 -/- arteries. Studies will then document the presence of mitogens in injured wild type and MMP-9 -/- arteries by immuno-electron microscopy. Finally since our preliminary data suggest that MMP-9 may regulate SMC replication by clipping the ectodomain of cadherin, we will evaluate the localization of beta-catenin in SMC isolated from wild type and MMP-9 -/- arteries. We will also measure activation of a beta-catenin/ LEF1 reporter construct in wild type and MMP-9 -/- cells since this is known to upregulate cyclin D1 expression. The final study will be to provide evidence of activation of the beta-catenin/LEF1 site in injured wild type and MMP-9 -/- arteries.